My MX's dead 12V battery in Vermont apparently requires a 173-mile tow to Latham, NY.

[MichaelHill]

The battery is beyond being jump-started ... even by a Tesla-approved VT-based towing company. It was only thru the Towing Company that I learned that Tesla has a VT-based mobile service team (out of Burlington, less than 40 miles away). Still, Tesla Emergency Roadside Assistance says that rather than pay for a VT-based jump or replacement, I need to have the X towed 173 miles to nearest Service Ctr. (in Latham, NY). Late-provided excuse is the Burlington-team "does not have a 12V battery" ... no reason why they (or I) couldn't buy one rather than suffer a 173-mile tow to a place that itself can't guarantee it has the part.

I moved to VT (from MD) in August and will likely sell the X rather than keep it in a place where even a battery-replacement requires a 173-mile tow.

 

[DCGOO]

The battery is beyond being jump-started ... even by a Tesla-approved VT-based towing company. It was only thru the Towing Company that I learned that Tesla has a VT-based mobile service team (out of Burlington, less than 40 miles away). Still, Tesla Emergency Roadside Assistance says that rather than pay for a VT-based jump or replacement, I need to have the X towed 173 miles to nearest Service Ctr. (in Latham, NY). Late-provided excuse is the Burlington-team "does not have a 12V battery" ... no reason why they (or I) couldn't buy one rather than suffer a 173-mile tow to a place that itself can't guarantee it has the part.

I moved to VT (from MD) in August and will likely sell the X rather than keep it in a place where even a battery-replacement requires a 173-mile tow.

I had the "replace 12v battery soon" alarm pop up in early December, at roughly 3:00 PM. I immediately opened a service ticket from the app. I live only a few miles from my local Service Center, but mobile service came to my home the very next morning and replaced it. According to the tech, 12v battery replacements are considered a very high priority when they come in. Mainly because it is a simple fix, and a complete failure often means a tow.

What I find odd in your case, is that they didn't have a 12v battery in stock, and/or could not get one quickly. The 12v battery has been replaced twice on my 2018 X.

 

[Solarman004]

@MichaelHill the battery replacement in the MX is time consuming, but not difficult. It's a straight forward DIY project and you can find detailed instructions online, and detailed YouTube videos.
Not an ideal solution, but better than a 173 mile tow, is to drive the 173 miles (assuming you have access to another car), buy the battery at the service center and install it yourself. For our 2016 MX I had to do this before a SC opened nearby, but my drive to pick up the battery was only 1.5 hours each way.
If go the DIY route, pay close attention to the unique Tesla steps such as disconnecting the high voltage loop, and making sure your front windows are open before disconnecting the battery.
And you can join the rest of us, cursing under your breath, asking the question "why didn't Tesla just use a darn 12 V battery that I can pick up at AutoZone?"

 

[ATPMSD]

I am a bit perplexed by this. If you connect a good 12v battery via a jumper to your dead one you should be able to power up the car , When the car "starts" the car's the high-voltage relay closes and the car comes to life. Once the car is running on the main battery the 12v battery is not used.

Often when a car will not start, or fails while driving, something else has gone wrong - at least that is what I have often read in these forums. Let us know if a new battery solves the problem.

 

[David29]

I am a bit perplexed by this. If you connect a good 12v battery via a jumper to your dead one you should be able to power up the car , When the car "starts" the car's the high-voltage relay closes and the car comes to life. Once the car is running on the main battery the 12v battery is not used.

Often when a car will not start, or fails while driving, something else has gone wrong - at least that is what I have often read in these forums. Let us know if a new battery solves the problem.

"...the 12 v battery is not used?" I understand the 12V battery in Tesla cars and in virtually all other EVs actually drives most of the car's electrical loads -- electronics, lights, audio, wipers, etc., everything except the propulsion motors and the HVAC. What am i missing?

 

[ATPMSD]

"...the 12 v battery is not used?" I understand the 12V battery in Tesla cars and in virtually all other EVs actually drives most of the car's electrical loads -- electronics, lights, audio, wipers, etc., everything except the propulsion motors and the HVAC. What am i missing?

The 12v battery does not run the car, its primary purpose is to run a small number of systems when the car sleeps and is most importantly used to close the high voltage (HV) relay bringing the HV battery online. If the 12v battery is dead you cannot wake up the car or unlock it.

Once the HV system is online it powers a DC-DC transformer, which powers the 12v system and charges the battery, if needed. Other than the DC-DC converter, the HV powers

HVAC
Heater
Drive Units

And the 12V system powers the rest:
Lights
Audio
Screens
wipers
Powered brakes (without power, you can still mechanically brake but harder)
Powered steering (without power, you can still mechanically steer but harder)
Battery Management System (no 12V means no way to charge the main battery)
Small motors/solenoids/electronic latches: for seats, windows, doors...
Accessories...

 

[David29]

The 12v battery does not run the car, its primary purpose is to run a small number of systems when the car sleeps and is most importantly used to close the high voltage (HV) relay bringing the HV battery online. If the 12v battery is dead you cannot wake up the car or unlock it.

Once the HV system is online it powers a DC-DC transformer, which powers the 12v system and charges the battery, if needed. Other than the DC-DC converter, the HV powers

HVAC
Heater
Drive Units

And the 12V system powers the rest:
Lights
Audio
Screens
wipers
Powered brakes (without power, you can still mechanically brake but harder)
Powered steering (without power, you can still mechanically steer but harder)
Battery Management System (no 12V means no way to charge the main battery)
Small motors/solenoids/electronic latches: for seats, windows, doors...
Accessories...

Interesting and important clarification. What you are describing is that the 12V battery is connected to the 12v system in parallel with the DC-DC transformer, whereas I had always understood (and perhaps this was more of an interpretation rather than an actual bit of knowledge) that the battery was in series with the downstream consumers, hence the system would not work without it. I wonder, though, if the 12v system also needs the 12v battery to stabilize the voltage level, much like on an ICE car?

 

[ATPMSD]

I wonder, though, if the 12v system also needs the 12v battery to stabilize the voltage level, much like on an ICE car?

I would guess not. If the 12v battery is dead you can use a jumper to supply power to the 12v system and then wake up the car, at which point it is then drivable.

Also, as part of the battery replacment process you must disconnect the HV loop to ensure no power is being supplied to the 12v system. In some videos when this is done after disconnecting the 12v battery, if the HV relay was closed you hear the HV relay open.